1. Technical Field
The present invention relates to the technical field of conversion of analog signals into digital signals and in particular relates to an analog/digital converter and a method for calibrating said converter.
2. Description of the Related Art
In many electronic apparatuses, the need is felt to convert analog signals into digital signals. A significant example of this type of apparatus is given by the digital terminals of mobile communication. In these applications, the requirements for low consumption, low voltage and high performance create particularly severe conditions for the design of the integrated circuits which make up the electronic system of the terminal.
The possibility of producing said integrated circuits with an ever-increasing level of integration (e.g. pure CMOS technology) leaves a greater area available on the chip which can be used for implementing digital signal processing (DSP) techniques often directed at improving the performance of analog components/circuits, for example used to compensate for imprecision inherently introduced by analog components/circuits.
This trend is also present in the sector of analog/digital converters, in particular in the sector of multistage converters, for which correction methods have been developed, more commonly known as calibration methods, intended to correct errors present in the conversion result due to the non-ideality of analog components, such as capacitors or operational amplifiers, present in the converter. Said non-ideality is, for example, attributable to the capacitor mismatch of the sampling circuits present at the converter input and to the limited gain-bandwidth product of the real operational amplifiers.
Among the calibration methods which belong to the prior art, there is a first type of method which makes it possible to perform a foreground calibration, in other words calibration performed when the analog signal to be converted is not applied to the converter input. These methods have the advantage that they have only a limited circuit complexity and the advantage that they guarantee an accurate conversion result. However, the foreground calibration methods have the intrinsic drawback that they cannot be used when the analog signal to be converted is applied to the converter input. In some applications however continuous calibration during signal conversion is necessary.
To overcome this drawback, a second type of calibration method has been developed. In particular, these methods use correlation methods and they are able to operate during conversion of the analog signal without interfering with the normal operation of the analog/digital converter.
These methods normally work by adding, to the analog samples of the signal to be converted, respective samples of a pseudorandom noise sequence, so that the error to be corrected is modulated by said sequence. The digital output of the converter is processed by means of correlation operations so as to extract the modulated information with the aim to improve the performance of the converter. It has been empirically proved that between the convergence time tconv (time to reach half LSB precision) of said correction methods and the resolution N in number of bits of the converter, there exists a relation of the type:tconv=22·NTs 
where Ts is the sampling period.
From the above relation it can be seen that, for example, by using one of the above-described background correction methods of the prior art in a cyclic converter with a resolution of 13 bits and operating at a frequency of 5 MS/s (mega-symbols per second), a convergence time of approximately 30 seconds is obtained. This extended convergence time makes the background calibration methods of the known art unsuitable for use in numerous applications.